The present invention relates to novel polymer film bilayers for non mixing contact of fluids, heat exchangers comprising such bilayers, the preparation and uses thereof, in particular to polymer film heat exchangers having a favourable ratio of heat exchanging surface area to path length of gas and/or liquids, processes for the preparation thereof and applications thereof.
In recent years polymers have become available with working temperatures up to and beyond 250.degree. C. Some typical examples are polyether ether ketone (PEEK) and poly imide (UPILEX). Since the chemical and fouling resistance of these materials is usually attractive, there is an emerging opportunity for them to displace metals in the construction of heat exchangers which operate at .ltoreq.250.degree. C.
The thermal conductivity of polymers is poor compared with that of metals so it is important that the thickness of polymer used does not impose a significant extra thermal resistance upon the heat transfer process. This implies that polymer film having a thickness of 100 microns is typically an appropriate choice for a gas-liquid heat transfer duty. The film is corrugated so that adjacent film layers may be held apart by the corrugations which cross at approximately 90.degree.. Where heat is being transferred from fluid A to fluid B, these fluids flow alternately through layers of corrugated film.
As a result of exploratory studies recently completed, it is now known that the heat transfer performance of a simple corrugated film matrix is very high using corrugations.having a peak to peak dimension of approximately 2 mm and a corrugation height of approximately 1 mm. However, in view of the relatively high pressure drop per unit flow path length, compared with that encountered in conventional metal heat exchangers, the flow paths must be minimised. This requirement applies particularly to the gas flow path length, when heat is being transferred between gases and liquids, eg in a domestic gas water heater. Recent ("condensing") versions of these heaters include secondary heat exchangers which cool combustion products from 250.degree. C. to 50.degree. C. approximately, thereby improving their overall thermal efficiency. Since the condensate produced in these units is weakly acidic the secondary heat exchanger is constructed in stainless steel. This entails a significant extra cost compared with the usual cast iron unit and provides an opportunity for a relatively cost effective construction in polymer film.
However, due to the limited flue gas pressure drop available, a secondary heat exchanger based upon a polymer film matrix, with its narrow flow channels, must involve a gas flow path length of only 10 cm or so. This invention is concerned with a novel design for achieving that requirement economically. While the design is particularly relevant to flue gas cooling with liquids, it also has significance for any gas-liquid heat transfer operation where the gas pressure drop is restricted.